Wasting and stunting are common in breastfed infants under six months in India and elsewhere but we know little about the prevention of these conditions. This research will generate critical knowledge about maternal malnutrition and lactation performance that will guide future maternal and child interventions. The mother should be the only nutrition portal to reach the young breastfed infant. Breast milk is undeniably the optimal source of vital nutrients and maternal immunoglobulins required for infant health and survival; however for too long the assumption has been that poor, thin women in India can produce as much high quality milk as the child demands. Fear of opening the door to inappropriate breast milk substitutes has discouraged research about potential effects of maternal malnutrition on breast milk. What little research exists often suffers from methodological limitations (such as small sample sizes and reliance on test weighing). We plan to enroll 232 lactating women with children 2-4 months of age. We propose preliminary work to allow us to assess the feasibility of using deuterium oxide (dose-to-mother technique) to measure milk volume and body composition in rural India. Our hypothesis is that the poor nutritional status of Indian women contributes to suboptimal lactation performance such that the child's high nutrient requirements are not met. In addition, we plan to explore whether maternal malnutrition leads to an altered inflammatory and immune milk profile and impaired infant intestinal permeability; which would further place the child at risk for early growth failure. Our key hypotheses are: Hypothesis 1: Women in rural India produce less milk and of lower energy and micronutrient density than necessary to meet children's optimal intakes. Hypothesis 2: Maternal malnutrition (low BMI, % body fat, mid upper arm circumference, and suboptimal micronutrient status) is associated with decreased breast milk quality (milk fat %, quantity and inadequate micronutrient concentrations) Hypothesis 3: Maternal malnutrition is associated with an inflammatory milk profile (higher Na:K ratios and pro-inflammatory cytokines) and breast milk inflammation is associated with infant intestinal permeability. This innovative research will provide critical information to inform maternal-focused nutrition strategies. The end goal would be to make national recommendations for the improvement of maternal and child health and nutrition outcomes in India and elsewhere.